The present invention relates generally to a system and method which utilizes the colorpatches of a colorbar for both color control and ink register control.
The field of web offset printing has seen great benefit from the revolution in computer electronics. As a result of the ever decreasing cost of electronics, sensing technology, and particularly computing horsepower, tasks formerly done by pressmen are increasingly performed by machines. Because human perception or bias is no longer involved in many of the quality control functions of printing, the consistency of the printing operation is increased. In turn, publishers have taken advantage of this increased consistency to minimize the amount of wasted paper that formerly was allocated for quality control purposes.
Some quality concerns revolve around the characteristics of the inks applied to a web which is typically paper. These ink characteristics include the strength or saturation of the ink (which is controlled by the thickness of the ink film, in turn controlled by ink dosing mechanisms within the print units), trap (which is a measure of the ability of an ink to be printed on top of a previously printed ink as compared to being printed on uninked paper), slur or doubling (which appears as a smearing of the ink), register (which is the relative positions of the inks to each other and is also known as print-to-print register or color register), backup or through-the-sheet register (which is the positions of the inks on the top side of the sheet relative to the bottom side of the sheet), and print-to-cut register or cutoff (which is the relative positions of the ink to the position where the web is cut into sheets).
Areas of the web are generally allocated for quality control purposes and include form ID numbers, laps which are used to open a folded form for saddle-stitching, cutoff control targets used to ensure that the web is properly cut into sheets at the correct position, color register targets used by a color register control system to assure proper alignment of the various colors relative to each other, and colorbars used to verify and control ink feed to the printing units.
Cutoff control targets used in a cutoff control system are generally contrasting rectangular marks placed on a lap. The position of the marks is determined and compared to their respective desired position. Web compensators are commanded to move in a manner to adjust any circumferential or print-to-cut register error. As a result, the web is properly cut into sheets, and the printed material on the sheet registers to the cut edge.
Color register targets for a color registration system are generally full-tone small marks of the individual inks of predetermined shape, such as dots, squares, triangles, or diamonds. The positions of the marks relative to each other are determined and compared with desired positions to maintain the respective colors in proper relative alignment. Color register control is differentiated from color density control in that the former controls the positions of the inks with respect to one another and the latter controls the ink film thickness, the strength or the saturation of the inks applied to the web.
A colorbar, used for monitoring color quality, is a lateral sequence of small colorpatches, typically rectangular, of the various color inks printed substantially fully across the web in a direction perpendicular to the direction of web travel. The colorpatches are printed in varying combinations, and have been used for measuring ink density or controlling the dosage of ink to the various alleys of the web, and optionally for measuring or controlling other ink characteristics. An alley or key area is a circumferentially extending strip of the web that is inked by a particular ink dosing mechanism or ink key. Numerous alleys comprise the printable surface of the web. A typical method used to measure the optical density of the ink utilizes either a densitometer off-line of the web printing process or a color video camera on-line.
In lower quality printing, such as newspaper printing, a halftone overprint of the cyan, magenta, and yellow inks is printed as a bar, often near a reference bar of black ink. Since newspapers are not typically trimmed, such a bar may be camouflaged as a part of the masthead. The bars typically appear gray and variations from gray in the balance of the color are usable for color control purposes.
Typically, in high quality web offset printing, ink colorpatches are printed at full-tone or 100% strength, at 75% and 50% halftone strengths, and in various patterns to measure such parameters as color strength, trap, print contrast, slur, and dot gain.
Various closed-loop printing press quality control systems are shown in U.S. Pat. Nos. 4,885,785, 4,887,530, 5,412,577, 5,689,425, 5,724,259 and 5,967,050.
The cutoff control system described in U.S. Pat. No. 4,885,785 is capable of using either a discrete cutoff mark, or the printed image itself, as the target whose position is determined. To reduce paper waste, the printed image itself is used rather than a discrete cutoff mark.
The register control system described in U.S. Pat. No. 4,887,530 uses discrete register marks. These discrete marks are typically placed on a lap, or if space is at a premium in a particular run, these marks are embedded in the colorbar, replacing some colorpatches in the colorbar.
The color measurement system described in U.S. Pat. No. 5,724,259 has been found to be accurate with a colorbar as narrow as {fraction (1/16)} inch high. The colorbar is typically placed at the position where the paper will be cut into individual sheets so that ordinarily {fraction (1/32)} of an inch of the colorbar will appear at the top of a sheet, and {fraction (1/32)} of an inch of the next colorbar will appear at the bottom of the sheet. Because more than {fraction (1/32)} of an inch of paper is typically trimmed from a sheet to form a final book or magazine, the colorbar incurs no additional paper waste. To minimize paper waste, the printed image is typically printed to abut directly to the edge of the colorbar. Combined with a system to control the ink-feed mechanisms of a press, a color measurement system becomes a color control system which controls the strength or saturation of the ink in the various alleys of the web. Such control of inking levels is well known and details may be found in U.S. Pat. Nos. 4,881,181 and 5,029,527.
A small colorbar has a disadvantage. When a press run is first started, the various colors of the printing units are usually misaligned or misregistered with respect to each other. If circumferential misregister causes the printed image to overlay or bleed directly into the top and bottom of the colorbar, with no bordering white space as a buffer, the colors of the misregistered image will contaminate the colors of the colorbar, preventing proper operation of the color control system. If lateral misregister causes the colorpatches to overlay each other, the sampled colors are similarly contaminated by each other. A certain minimum area of uncontaminated color is needed as a sufficient sample to accurately determine color density. Color measurement or control therefore cannot commence until the registration is manually or automatically performed. If register targets replace colorpatches in the colorbar, these targets may similarly be contaminated by bleed, preventing their recognition. In this situation both register and color control are inoperative, requiring manual intervention.
An abutted colorpatch in itself is not a reliable register target, since it is adjacent to, or in the case of circumferential misregister, partially overlaid by the printed image. Since the printed image may be of a similar color to the colorpatch, the colorpatch may have little contrast against the image, preventing the colorpatch""s edges, and therefore its position, from being accurately determined. Similarly, a dedicated register target in the colorbar is unreliable, unless it is small enough that its edges are not abutted by the printed image under worst-case misregister. Due to the limited resolution of printing, such a small target cannot have a complex shape, so there is a risk that the printed image may coincidentally have a similar misleading shape. The register system will malfunction if target misrecognition occurs.
The markless register control systems described in U.S. Pat. Nos. 5,412,577 and 5,689,425 use the printed image itself as the source of register information.
The web in a printing press is subject to lateral and circumferential shifting, especially at critical times of startup, so that any mark on the web may not be in an expected position. If a mark is small as is usually desired, and the control system images only a small area of the web, as is typically needed for adequate camera resolution, then searching for the mark is required. A strobe light can be used to image register marks or target areas on a small area of a web. If the marks are not found, a 2-dimensional search, i.e., in both the lateral and circumferential directions, is required because the marks could have moved in any direction. Searching for marks can be time consuming and costly.
Closed-loop printing press quality control systems are typically embodied in a structure or stand which contains the needed scanners, controls, and electronics. The size of the stand can vary considerably, and the control system components are typically mounted wherever free space is available. If a control system is to be retrofitted onto an existing press, the floor space may be unavailable. Floor space may be minimized by stacking the various components atop one another. Stacked components are less accessible for cleaning, web-up, or inspection. Each control system also requires regular maintenance because printing presses generate paper dust and tiny droplets of ink, both of which obscure the optics of the various scanners. To minimize floor space, U.S. Pat. No. 5,125,037 describes a single system for both color and register control purposes. This system uses large amounts of expensive white space between targets so that the problem of bleed does not occur, and does not disclose methods of recognizing a target buried in bleed. U.S. Pat. No. 6,109,183 also describes a single system for both register and color control purposes, and likewise does not disclose methods of recognizing a target buried in bleed.
One current trend in web offset printing is the use of wider web widths. A decade ago, the typical high-speed web offset press was capable of printing a 38-inch wide web, or four typical magazine pages wide. The current standard is to print a 54-inch wide web, or six magazine pages. A wider width emphasizes several errors common in offset lithography. One problem is called cocking register error wherein one ink color in the printed image is skewed slightly with respect to the others. Register of a color may be correct at the left side of the web but be offset vertically on the right side. This problem is corrected by skewing the plate cylinder of the press in the opposite direction. There are various causes for a cocking error including a printing plate being incorrectly imaged or installed, or uneven paper characteristics.
Another problem prevalent in wider webs is called fit register error in which the colors"" register will be correct at the center of the web, but the later-printed colors will misregister toward the edges of the web. This is due to an inherent part of lithography in that water and ink are used in the process, and water causes the paper web to widen. Because the web is wider at the last printing unit than the first, having absorbed more water, the later-printed colors are relatively narrower. This problem is addressed with the use of bustle wheels. The bustle wheels are mounted below the web and are adjusted to impinge upon the web creating a slight wrinkle. The bustle wheels are placed in a position, such as a fold or cut line, where the wrinkle will not be noticed in the final product. The wrinkle takes up paper laterally, approximately shrinking the paper back to proper size. On presses such as the Lithoman 64, produced by M. A. N. Roland of Augsburg, Germany, such bustle wheels are motorized, allowing remote operation.
Register systems which scan only a single set of register marks cannot determine fit or cocking register error. Comparison of register on one side of the web with the register on the other side of the web is needed to determine cocking and fit register error.
The preferred embodiment of the present invention utilizes colorpatches for both color control and register control. Color measurement steps are used to determine the color density of colorpatches, while the same colorpatches are used to determine the respective positions of each ink color. If the printed image bleeds into and contaminates the upper or lower edges of the colorbar, or if lateral misregister acts to partially overlay one colorbar atop an adjacent colorbar to the left or right, the positions of the colorbars can be accurately determined with respect to each other, corrections can be sent to the register-correcting motors of the printing units, and the misregister corrected. With correct register, the colorbars are not overlaid by the printed image or each other, allowing for accurate color measurement and control.
In the preferred embodiment, distinguishing attributes are embedded in the colorpatches and are left uninked to improve the determination of colorpatch position despite impingement of the colorpatch by image bleed or another colorpatch. Limitations of the prior art are avoided by a two-step process of recognition of the distinguishing attribute. First, an approximate determination of the position of the colorbar (and therefore the distinguishing article which is in a known positional relationship to the colorbar) is made. Second, an exact determination of the position of the distinguishing article is made. Since the approximate position of the distinguishing attribute is known with respect to the colorbar, only a small area need be examined to exactly determine position.
Because the entire width of the web is preferably scanned in normal operation to control the color of the various alleys of the web, fit and cocking register errors can also be determined. Since the same scanner is used for both color and register control, equipment complexity and maintenance duties are minimized.
Features and advantages of the invention will become apparent to these of ordinary skill in the art upon review of the following drawings, detailed description and claims.